Title: Comparative Spatial Spread over Time of Zucchini Yellow Mosaic Virus and Watermelon Mosaic Virus in Fields of Transgenic Squash Expressing the Coat Protein Genes of Zymv and Wmv, and in Fields of Nontransgenic Squash

Interpretive Summary: Experiments were conducted to determine if transgenic commercial squash that show resistance to watermelon mosaic virus 2 and zucchini yellow mosaic virus, but show localized chlorotic spots on some plants would serve as a source of virus inoculum under field conditions. Comparative studies with nontransgenic squash clearly showed that the transgenic plants do not serve as inoculum sources for virus spread. The results were documented by visual assessment of plants and by mathematical models. This work provides evidence that virus resistant transgenic squash describes does not pose an environmental risk to being a source of virus.

Technical Abstract:
The spatial and temporal patterns of aphid-vectored spread of Zucchini yellow mosaic virus (ZYMV) and Watermelon mosaic virus (WMV) were monitored over two consecutive years in plantings of nontransgenic and transgenic squash ZW-20H and ZW-20B, all surrounded by nontransgenic plants mechanically inoculated with ZYMV or WMV that served as primary virus source. Across all trials, none of the transgenic plants exhibited systemic symptoms upon infection by ZYMV and WMV but a few of them developed localized chlorotic dots and/or blotches and had low mixed infection rates [4% (6 of 139) of ZW-20H and 9% (13 of 139) of ZW-20B]. Geostatistical analysis of ELISA positive transgenic plants indicated a lack of spatial relationship on spread of ZYMV and WMV with flat omnidirectional experimental semivariograms that poorly fitted theoretical models. In contrast, a strong spatial dependence on spread of ZYMV and WMV was found for nontransgenic plants, which developed severe systemic symptoms, had prevalent mixed infection rates (62%, 86 of 139), and defined omnidirectional experimental semivariograms that fitted a spherical model. Geostatistical data were sustained by virus transmission experiments with Myzus persicae in screenhouses, showing that commercial transgenic squash ZW-20 alter the dynamics of ZYMV and WMV epidemics by preventing secondary plant-to-plant spread.